Suppose the marginal costs of exercising are constant and the marginal benefits of exercising decline over time. With marginal costs and benefits placed on the vertical axis and time placed on the horizontal axis, the resulting marginal cost curve would be a ____________________ line and the resulting marginal benefit curve would be _____________________.
a.horizontal, vertical
b.vertical, downward-sloping
c.horizontal, downward-sloping
d.horizontal, upward-sloping